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Lecture 3

BIOC50H3 Lecture Notes - Lecture 3: Stromatolite, Archean, Proterozoic


Department
Biological Sciences
Course Code
BIOC50H3
Professor
Jason Weir
Lecture
3

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Lecture Three
Precambrian: Includes Archean and Proterozoic
- From 38000 to 542 mya
- Earliest evidence of life:
o Comes from carbon granules and they are 37000 mya
They have a C13 and C12 ratio consistent with life
They are both stable isotopes but living organisms sequester C13 at a
different ratio than C12 that is found in the environment; suggest they
weren’t formed by natural processes
- Earth’s Earliest fossils
o Occurred through stromatolites
Mounds of sediments with alternative layers of cyanobacteria and other
bacteria
They are rock like structures in modern era
Can be found today in Australia and British Columbia
In the fossil record, they stretch back as far as 3,450 mya
Not everyone believes that these stromatolites represent evidence of life
3.1 bya…there were two distinct styles of bacterial mat…stromatolites
have diversified into two clades
By about 2.8 mya, they were known from both lake and ocean
environment
o Actual cells in the fossil records:
We have bacterial like fossils; they look like fossils dating back to almost
3.5 billion years ago; not 100% sure they are fossils
Cyano- bacteria like cells
One evidence that do they represent actual form of life is the C13/C12 in
the micro fossils suggest life
To look for a phylogeny of life:
- Look for last common ancestor
- You need to build a phylogeny tree based on morphology
- We lack a detailed fossil record
- An alternative approach is to reconstruct phylogenetic trees of all living things, and see if
it gives us clues to the common ancestor of all life
- The first attempt to reconstruct the tree of life used morphological data included five
kingdom scheme:
o Prokaryotes
o Fungi
o Plants
o Animals
o Protist
o Obviously wrong
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- What challenge will you have making a phylogeny tree based on morphology?
o Tree of life deals with bacteria, fungi, animals, protists…what do they have in
common:
There are not much morphological characters to use from them to actually
build a phylogeny tree
Better to use a phylogeny tree based on DNA:
What’s challenge about this?
o Not every organism will have the same genes; we need to
search for genes that are all universally common
o You need to find genes that are conserved and preserved in
all living forms…we do actually have some genes
o Any DNA/ Protein used to generate a phylogenetic tree of all of life must come
from a gene found in all living species
o One such gene is small subunit ribosomal RNA used in ribosomes; all living
things have the small subunit ribosomal RNA
Therefore we can use RNA to make a phylogeny tree
o Using these universal genes, we can construct a tree of life
There are three major group: Bacteria, Archaea, and Eukarya
Where do we draw the roots? When we root a tree, we get an
outgroup that’s distantly related to the in group
However this is a tree of ALL of life and we don’t have an
outgroup
Answer: We can use a gene duplication events that make gene
copies…it predates the origins of the common ancestor of life
formed gene families…used to root the universal phylogeny
o Gene copies helps root the tree
Root shows that bacteria splits off first; and that archaea and eukaryote are
more closely related
- The last common ancestor of life:
o About 590 mya fossil (preserved cell wall…it is a single celled eukaryote,
reproductive cyst of multicellular alga
o We also have a 900 mya fossil
o Then we have an algae alike creature known as Grypania…goes back to almost
2.1 Bya; oldest Eukaryotes fossil
Probably an alga
- Looking at the actual bacterial side:
o Oldest bacteria: Cyanobacteria
Left side: extant bacteria
Right side: Oldest type of fossil found
- This helps to narrow down the line of common ancestor:
o About 14000 to 2100 mya: latest possible date for the split between Archaea and
Eukaraya
o 2000 Mya: latest possible date for the split between bacteria and archaea
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